Transmission line load for high frequencies



1944- I H. E. GCjJLDSTINE 2,354,809

TRANSMISSION LINE LOAD FOR HIGH FREQUENCIES Filed Sept. 18, 1942 I TLE LZ5 16 f/ 10 Z3 Z2 lllw H I l W 15 Z6 33 v INVENTOR ATTORI'NEY PatentedAug. 1, 1944 2,354,309 mNsmssIoN mm LOAD Fon man moms FREQ

7' Hallan 1:. Goldstine, Rocky Point, N. Y., assignor to RadioCorporation of America, a corporation of Delaware Application September18, 1942, Serial No. 458,801

' 10 Claims. (Cl. 201-64) The present invention relates to dummy loadsfor high frequencies and, particularly, for loads designed to matchcoaxial transmission lines over a wide band of frequencies.

An object of the present invention is the provision of a high frequencyload which is compact in size.

Another object of the present invention is the provision of a highfrequency load in which the power dissipated may be readily determined.

ing a diameter of .004 inch was used for the in- A further object of thepresent invention isto provide a high frequency load which is aperiodicin its aracteristics.

St a further object of the present invention is th provision of a highfrequency load which is capa e of working over a wide band of frequen--cles without the necessity of returning the apparatus for eachfrequency.

A further object of the present invention is the provision of a highfrequency load in which a considerable amount of power can be dissipatedwithout materially changing the electrical characteristics.

It is desirable when adjusting transmitters to determine thecharacteristics of the transmitter when working into a resistive load. Aload having the proper resistance will absorb all of the energy appliedthereto through a transmission line connected to the transmitter. If theload is so designed as to have an impedance equal to the surge impedanceof the transmission line there will be no reflected wave and the transmission line is said to be matched to the load. For high frequencies thesurge impedance of low loss transmission lines is substantiallyresistive.

For adjusting the loading of a transmitter, its

modulation characteristics, etc., it is essential that the transmitteroperate into a transmission line which is properly terminated.

Therefore, in accordance with the principles of the present inventionthe load which is used may be constructed in the form of a concentricline having a resistance equal to the surge impedance of thetransmission line to which it is connected and having the furtherproperties that its input impedance is purel resistive and does notchange with frequency. A small concentric line, the inner conductorhaving a high resistance per unit length, may be used for the load,

The present invention will be more fully understood by reference to thefollowing detailed description which is accompanied by a drawing inwhich Figure 1 shows one view of a dummy load constructed with theprinciples of the lnvenner conductor i3. The outer conductor H was acopper tube having an inner diameter of .023-

inch and an outer diameter of .028 inch. The inner and outer conductorswere maintained in their proper coaxial relationship by spun glassinsulation l5 which was wrapped around the inner conductor I: in theform of a close helix of several layers. The surge impedance Z0 of sucha transmission line is given by the following relationship;

G-l-jwC' (1) where R is the resistance and L and C the inductance andcapacity per unit length and G is the conductance per foot.

The capacity per foot of a transmission line having the dimensions asgiven above is of the order of 26 ,uaf. per foot while the inductance isabout .106 th. per foot. The resistance per foot is approximately 43ohms. The above given value of inductance is determined from therelationship h/ft.=61 10g. 5 x10 At 15b megacycles,- neglecting the termG of,

Equation 1, since its effect is small, the surge impedance phase angleis smaller, as shown by the following example:

By the correct selection of wire size and insulation thickness the surgeimpedance may be made to match any transmitter transmission line.

The attenuation is so high on the load line that short piece of linerepresents an input impedance that will not vary greatly with a changein frequencies. If the section of coaxial line which constitutes thedummy load is open circuited on the far end of the line the inputimpedance Zin is given by the following:

sinh 2aL-j sin 25L, cosh 2aL--COS 25L :here or is the attenuationconstant in nepers per unit length and B is the wavelength constant inradians per unit length. If Z=R+jiwL and Y=G+iwC, 'y (propagationconstant) =x fi, the real part of which is equal to a and the imaginarypart equal 5,

With the constants given above for the small nichrome concentric linethe input impedance varies about :4%, if the line has a length about 6feet for an operating frequency of 150 me.

A convenient mechanical arrangement of the load is to wind the coaxialline H! into a small diameter helix l5, as shown in section in Figure 2.With the successive turns touching, and using such small line dimensionsa very compact coil results. For example, the 6 feet of line mentionedabove may be wound into a. 1 inch diameter helix having about 23 turns.The total height of the coil is then approximately .64

tion of time. Then calibrating power is applied to the conductor ll andthe amount of power required to give the same amount of temperature riseis noted. Thus the power dissipation at radio frequencies may bedetermined.

Due to the attenuation of the radio frequency power along the length ofline Ill, the power dissipation will not be equally distributed over thelength of the line. The first foot of the transmission line will havemore power dissipated than the second ioot, etc. Therefore, the linemust be soarranged that the insulation It is not excessively heated noris the inner conductor it burned out due to local overloads. This may beassured by using a lower specific resistance wire for the part nearestthe input end; or the length of the line may be made longer and theresistance of the wire lower if it is permissible to make the powerdissipation per unit of line length lower.

While I have particularly shown and described several modifications ofthe present invention, it should be clearly understood that theinvention is not limited to these forms alone, but that modificationsmay be made.

I claim:

1. A high frequency load including a two-conductor line having at leastone conductor with a high resistance per unit length for dissipating theenergy applied to said line there being no inch. The coiled coaxial linehas one end of inner conductor |3 connected to a plug ll of a coaxialline connector 20, while the casing 2| into which the helix is insertedis connected to the outer shell 22 of the coaxial line connector 20. Theconductive portions of connector are maintained in coaxial relationshipby means of insulators 23 which, preferably, are little aflectcd byheat. Plug It has a portion 2l' between insulators 23 of greaterdiameter than the central holes in the insulators to maintain theinsulator spacing and to hold the plug in position against end thrust.One insulator rests against a shoulder in casing 2| while the other isheld in place by a split ring 33 in a groove in connector 22. Theconcentric line "I, coiled within the cavity in casing 2| is embedded ina slug 2! of some metal of high heat conductivity such as tin or zinc. Athermometer 26 is inserted in the metal slug for measuring thetemperature rise under load. Casing 2| may further be provided withradiating fins 28 for dissipating the heat developed. In some cases itmay be desirable to employ forced air cooling or fluid circulationcooling. The open end of conductor |3 is connected to a terminal 30which is supported in position on casing 2| by an insulator 3|. Thecalibrating power, which may be direct current or low frequencyalternating current, may be applied to the conductor l3 by means ofterminal 36 and plug I8.

In use, the temperature rise is measured by means of thermometer 28 andrecorded as a fLll'lQ-v electrically conducting path between saidconductors.

2. A high frequency load including a two-conductor line having at leastone conductor with a high resistance per unit length for dissipating theenergy applied to said line and means for coupling a source of highfrequency energy to one end of said line, said line being open at theother end, the resistance of said conductor increasing along the lengththereof from said one end.

3. A high frequency load including a coaxial line having an outer sheathand an inner conductor, said inner conductor having a high resistanceper unit length for dissipating the energy applied to said line therebeing no electrically conducting path between said conductors.

4. A high frequency load including a coaxial line having an outer sheathand an inner conductor, said inner conductor having a high resistanceper unit length for dissipating the energy applied to said line andmeans for coupling a source of high frequency energy to one end of saidline, said line being open at the other end. the resistance of saidconductor increasing along the length thereof from said one end.

6. A high frequency load including a coaxial line having an outer sheathand an inner conductor, said inner conductor having a. high resistanceper unit length for dissipating the energy applied to said line. saidline being coiled into a helix and immersed in a substance of high heatconductivity.

8. A high frequency load including a coaxial line having an outer sheathand an inner conductor, said inner conductor having a high resistanceper unit length for dissipating the energy applied to said line, saidline being coiled into a helix and immersed in a substance of high heatconductivity and means for measuring the heat developed in said load.

'I. A high frequency load including a coaxial line having an outersheath and an inner conductor, said inner conductor having a highresistance per unit length for dissipating the energy applied to saidline and means for coupling a source of high frequency energy to one endof said line, the resistance of said conductor increasing along thelength thereof from said one end, said line being coiled into a helixand immersed in a substance of high heat conductivity.

8. A high frequency load including a coaxial line having an outer sheathand an inner conductor, said inner conductor having a high resistanceper unit length for dissipating the energy applied to said line andmeans for coupling a source of high frequency energy to one end of saidline, the resistance of said conductor increasing along the lengththereof from said one end, said line being coiled into a helix andimmersed in a substance of high heat conductivity and means formeasuring the heat developed in sistance per unit length for dissipatingthe energy applied to said line, said line being coiled into ahelix andarranged within a casing, said coiled helix being immersed in asubstance of high heat conductivity and means associated with saidcasing for removing therefrom the heat developed in said line.

10. A high frequency load including a coaxial linehaving an outer sheathand an inner conductor, said inner conductor having a high resistanceper unit length for dissipating the energy applied to said line, saidline being coiled into a helix and arranged within a casing, said coiledhelix being immersed in a substance of high heat conductivity and meansassociated with said casing for removing therefrom the heat developedinsaid line, said last mentioned means including cooling fins on saidcasing.

HALPAN E. GOLD STINE.

